Methods and devices for heating fluid in fluid enhanced ablation therapy

ABSTRACT

Devices and methods for efficiently and reproducibly heating fluid for use in fluid enhanced ablation are disclosed herein. In one embodiment, an ablation device is provided having an elongate body, at least one wire extending through an inner lumen of the elongate body, and at least one spacer disposed within the inner lumen. The at least one wire extends through the at least one spacer such that the at least one spacer is effective to maintain an adjacent portion of the at least one wire in a substantially fixed geometric relationship with the inner lumen, thereby preventing electrical shorts and providing for the consistent and uniform heating of fluid flowing through the inner lumen of the elongate body.

RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application Ser.No. 61/474,574, filed on Apr. 12, 2011, and entitled “Improvement inAblation Catheters.” This application is also related to U.S.application Ser. No. ______ entitled “Devices and Methods for RemoteTemperature Monitoring in Fluid Enhanced Ablation Therapy,” U.S.application Ser. No. ______ “Devices and Methods for Shaping Therapy inFluid Enhanced Ablation,” U.S. application Ser. No. ______ “Methods andDevices for Controlling Ablation Therapy,” and U.S. application Ser. No.______ “Methods and Devices for Use of Degassed Fluids with FluidEnhanced Ablation Devices,” respectively, and filed concurrently withthe present application. The disclosures of each of these applicationsare hereby incorporated by reference in their entirety.

FIELD

This invention relates generally to fluid enhanced ablation, such as theSERF™ ablation technique (Saline Enhanced Radio Frequency™ ablation).More particularly, this invention relates to improved devices andmethods for heating assemblies used to heat fluid introduced into tissueduring ablation.

BACKGROUND

The use of thermal energy to destroy bodily tissue can be applied to avariety of therapeutic procedures, including the destruction of tumors.Thermal energy can be imparted to the tissue using various forms ofenergy, such as radio frequency electrical energy, microwave or lightwave electromagnetic energy, or ultrasonic vibrational energy. Radiofrequency (RF) ablation, for example, can be effected by placing one ormore electrodes against or into tissue to be treated and passing highfrequency electrical current into the tissue. The current can flowbetween closely spaced emitting electrodes or between an emittingelectrode and a larger, common electrode located remotely from thetissue to be heated.

One disadvantage with these techniques is that maximum heating oftenoccurs at or near the interface between the therapeutic tool and thetissue. In RF ablation, for example, maximum heating can occur in thetissue immediately adjacent to the emitting electrode. This can reducethe conductivity of the tissue, and in some cases, can cause waterwithin the tissue to boil and become water vapor. As this processcontinues, the impedance of the tissue can increase and prevent currentfrom entering into the surrounding tissue. Thus, conventional RFinstruments are limited in the volume of tissue that can be treated.

Fluid enhanced ablation therapy, such as the SERF™ ablation technique(Saline Enhanced Radio Frequency™ ablation), can treat a greater volumeof tissue than conventional RF ablation. The SERF ablation technique isdescribed in U.S. Pat. No. 6,328,735, which is hereby incorporated byreference in its entirety. Using the SERF ablation technique, saline ispassed through a needle and heated, and the heated fluid is delivered tothe tissue immediately surrounding the needle. The saline helpsdistribute the heat developed adjacent to the needle and thereby allowsa greater volume of tissue to be treated with a therapeutic dose ofablative energy. The therapy is usually completed once a target volumeof tissue reaches a desired therapeutic temperature, or otherwisereceives a therapeutic dose of energy.

During fluid enhanced ablation therapy, the fluid can be heated to adesired temperature in a variety of different ways. For example, thefluid can be heated remotely from the needle and then pumped into theneedle at an elevated temperature. However, transferring heated fluidcan result in undesirable temperature loss between the remote heater andthe treatment site, as well as undesirable heating of remote portions ofthe patient's body. Alternatively, the fluid can be heated after itenters the needle and prior to injection into the tissue. However, itcan be difficult to construct and repeatedly manufacture a heatingassembly capable of disposition within the sometimes very small needlebodies used in fluid enhanced ablation. Furthermore, the needle bodyitself is a conductive material used to deliver energy to the treatmentsite, so precautions must be taken to avoid interfering with energypassed through the needle body.

Accordingly, there remains a need for improved devices and methods forheating fluid used during fluid enhanced ablation therapy.

SUMMARY

The present invention generally provides devices and methods forreliably and uniformly heating fluid for use in fluid enhanced ablationdevices. In one aspect of the invention, an ablation device is providedthat includes an elongate body having proximal and distal ends, an innerlumen extending through the elongate body, and at least one outlet portformed in the elongate body that is configured to deliver fluid totissue surrounding the elongate body. The device also includes at leastone wire extending through the inner lumen, the at least one wire beingconfigured to heat fluid flowing through the inner lumen, and at leastone spacer disposed within the inner lumen. The at least one spacer iseffective to maintain an adjacent portion of the at least one wire in asubstantially fixed geometric relationship with the inner lumen.

The ablation device described above can have a variety of modificationsthat are within the scope of the invention. For example, in someembodiments, the device can also include an ablation element disposedalong a length of the elongate body adjacent to the at least one outletport, and the ablation element can be configured to heat tissuesurrounding the ablation element when the elongate body is inserted intotissue. In other embodiments, the at least one wire and the at least onespacer can be positioned proximal of the ablation element.

In certain embodiments, the at least one spacer can include a firstspacer and a second spacer. The first spacer can be positioned at aproximal end of a distal portion of the at least one wire, and thesecond spacer can be positioned at a distal end of the distal portion ofthe at least one wire. In some embodiments, the first and second spacerscan be positioned a distance apart from one another, and the distancecan be about 5 mm. In other embodiments, the distance can be about 2 mm.In some other embodiments, a portion of the at least one wire extendingbetween the first and second spacers can be configured to heat fluidflowing through the inner lumen.

In still other embodiments, the at least one spacer can include adisc-shaped member and the at least one wire can include first andsecond wires configured to extend through first and second bores in thespacer. In some embodiments, the at least one spacer can include atleast one protrusion formed on the at least one wire.

In some other embodiments, the at least one spacer can prevent the atleast one wire from contacting the inner lumen of the elongate body. Incertain embodiments, the at least one wire can be insulated proximal tothe at least one spacer to prevent contact with the inner lumen of theelongate body. In still other embodiments, the inner lumen can be linedwith an insulating layer to prevent the at least one wire fromcontacting an inner wall of the elongate body.

In other embodiments, the at least one spacer can have a maximum outerdiameter that is less than a diameter of the inner lumen such that theat least one spacer can move radially within the inner lumen. In stillother embodiments, the at least one spacer can have a maximum outerdiameter equal to a diameter of the inner lumen such that the at leastone spacer cannot move radially within the inner lumen. In certainembodiments, the at least one spacer can be configured to maintain theat least one wire in a position substantially coaxial with alongitudinal axis of the elongate body.

In some embodiments, the ablation device can further include at leastone temperature sensor disposed within the inner lumen distal to the atleast one spacer and configured to measure a temperature of the fluidflowing through the inner lumen. In some other embodiments, the ablationdevice can further include a second temperature sensor disposed withinthe inner lumen proximal to the at least one spacer and configured tomeasure a temperature of the fluid flowing through the inner lumenbefore being heated. In some embodiments, the temperature sensors can bethermocouples. In still other embodiments, the at least one temperaturesensor can be separated from the at least one spacer by a distance ofabout 10 mm. In other embodiments, the at least one temperature sensorcan be separated from the at least one spacer by a distance of about 2mm.

In another aspect of the invention, a method of ablating tissue isprovided that includes inserting an elongate body into a tissue mass anddelivering fluid through an inner lumen of the elongate body such thatthe fluid flows through at least one outlet port in the elongate bodyand into the tissue mass. The method can further include deliveringenergy through at least one wire extending through the inner lumen touniformly heat the fluid within the lumen to a predeterminedtemperature.

In some embodiments, delivering energy through at least one wire caninclude passing energy between two or more wires extending through theinner lumen. In other embodiments, delivering energy through at leastone wire can include passing energy between one or more wires and theelongate body or between one or more wires and a conductive tubecontained within the elongate body. In still other embodiments, themethod can further include delivering energy into the tissue mass fromat least one ablation element positioned adjacent to the at least oneoutlet port.

In another aspect of the invention, a method for manufacturing aplurality of ablation devices is provided that includes forming a firstablation device by positioning a first heating assembly within a firstelongate body, the first heating assembly having at least one wireextending through at least one spacer. The method can further includeforming a second ablation device by positioning a second heatingassembly within a second elongate body, the second heating assemblyhaving at least one wire extending through at least one spacer. Further,the electrical resistance of the first heating assembly can besubstantially identical to the electrical resistance of the secondheating assembly.

In still another aspect of the invention, an ablation device is providedthat includes an elongate body having proximal and distal ends, an innerlumen extending through the elongate body, and at least one outlet portformed in the elongate body configured to deliver fluid to tissuesurrounding the elongate body. The device also includes a heatingassembly that includes at least two wires extending through the innerlumen, the at least two wires being configured to heat fluid flowingthrough the inner lumen. Further, the device includes at least onespacer disposed within the inner lumen. The at least two wires extendthrough the at least one spacer such that the at least one spacer iseffective to maintain the at least two wires in a substantially fixedgeometric relationship with each other. In some embodiments, the devicecan further include an ablation element disposed along a length of theelongate body adjacent to the at least one outlet port, and the ablationelement can be configured to heat tissue surrounding the ablationelement when the elongate body is inserted into tissue.

BRIEF DESCRIPTION OF THE DRAWINGS

The aspects and embodiments of the invention described above will bemore fully understood from the following detailed description taken inconjunction with the accompanying drawings, in which:

FIG. 1 is a diagram of one embodiment of a fluid enhanced ablationsystem;

FIG. 2 is a perspective view of one embodiment of a medical devicehaving an elongate body for use in fluid enhanced ablation;

FIG. 3 is a graphical representation of simulated heating profiles forvarious forms of ablation;

FIG. 4 is a diagram of one embodiment of an elongate body having adual-wire heating assembly;

FIG. 5A is a cross-sectional diagram of the elongate body of FIG. 4proximal to a spacer element;

FIG. 5B is a cross-sectional diagram of the elongate body of FIG. 4 at alocation of a spacer element;

FIG. 5C is a cross-sectional diagram of the elongate body of FIG. 4 at alocation between the spacer elements;

FIG. 6 is an exploded perspective view of one embodiment of an elongatebody having a dual-wire heating assembly;

FIG. 7 is a diagram of one embodiment of an electrical circuit fordriving the elongate body and heating assembly of FIG. 4;

FIG. 8 is a diagram of one embodiment of an elongate body having asingle-wire heating assembly;

FIG. 9A is a cross-sectional diagram of the elongate body of FIG. 8 at alocation adjacent to a spacer element;

FIG. 9B is a cross-sectional diagram of the elongate body of FIG. 8 at alocation of a spacer element;

FIG. 10 is a diagram of one embodiment of an electrical circuit fordriving the elongate body and heating assembly of FIG. 8;

FIG. 11A is a diagram of an alternative embodiment of a spacer elementformed on a wire;

FIG. 11B is a cross-sectional diagram of the wire and spacer element ofFIG. 11A;

FIG. 12 is a diagram of an alternative embodiment of a spacer elementformed on a wire;

FIG. 13A is a cross-sectional diagram of the elongate body of FIG. 4 ata location adjacent to a spacer element showing simulated lines ofcurrent flux; and

FIG. 13B is a cross-sectional diagram of the elongate body of FIG. 8 ata location adjacent to a spacer element showing simulated lines ofcurrent flux.

DETAILED DESCRIPTION

Certain exemplary embodiments will now be described to provide anoverall understanding of the principles of the devices and methodsdisclosed herein. One or more examples of these embodiments areillustrated in the accompanying drawings. Those skilled in the art willunderstand that the devices and methods specifically described hereinand illustrated in the accompanying drawings are non-limiting exemplaryembodiments and that the scope of the present invention is definedsolely by the claims. The features illustrated or described inconnection with one exemplary embodiment may be combined with thefeatures of other embodiments. Such modifications and variations areintended to be included within the scope of the present invention.

The terms “a” and “an” can be used interchangeably, and are equivalentto the phrase “one or more” as utilized in the present application. Theterms “comprising,” “having,” “including,” and “containing” are to beconstrued as open-ended terms (i.e., meaning “including, but not limitedto,”) unless otherwise noted. The terms “about” and “approximately” usedfor any numerical values or ranges indicate a suitable dimensionaltolerance that allows the composition, part, or collection of elementsto function for its intended purpose as described herein. These termsgenerally indicate a ±10% variation about a central value. Componentsdescribed herein as being coupled may be directly coupled, or they maybe indirectly coupled via one or more intermediate components. Therecitation of any ranges of values herein is merely intended to serve asa shorthand method of referring individually to each separate valuefalling within the range, unless otherwise indicated herein, and eachseparate value is incorporated into the specification as if it wereindividually recited. All methods described herein can be performed inany suitable order unless otherwise indicated herein or otherwiseclearly contradicted by context. The use of any and all examples, orexemplary language (e.g., “such as”), provided herein is intended merelyto better illuminate the invention and does not impose a limitation onthe scope of the invention unless otherwise claimed. No language in thespecification should be construed as indicating any non-claimed elementas essential to the practice of the invention. Further, to the extentthe term “saline” is used in conjunction with any embodiment herein,such embodiment is not limited to the use of “saline” as opposed toanother fluid unless explicitly indicated. Other fluids can typically beused in a similar manner.

Fluid Enhanced Ablation Systems

The present invention is generally directed to heater elements used influid enhanced ablation devices. Fluid enhanced ablation, as mentionedabove, is defined by passing a fluid into tissue while deliveringtherapeutic energy from an ablation element. The delivery of therapeuticenergy into tissue causes hyperthermia in the tissue, ultimatelyresulting in necrosis. This temperature-induced selective destruction oftissue can be utilized to treat a variety of conditions includingtumors, fibroids, cardiac dysrhythmias (e.g., ventricular tachycardia,etc.), and others.

Fluid enhanced ablation, such as the SERF™ ablation technique (SalineEnhanced Radio Frequency™ ablation) described in U.S. Pat. No. 6,328,735and incorporated by reference above, delivers fluid heated to atherapeutic temperature into tissue along with ablative energy.Delivering heated fluid enhances the ablation treatment because thefluid flow through the extracellular space of the treatment tissue canincrease the heat transfer through the tissue by more than a factor oftwenty. The flowing heated fluid therefore convects thermal energy fromthe ablation energy source further into the target tissue. In addition,the fact that the fluid is heated to a therapeutic temperature increasesthe amount of energy that can be imparted into the tissue. Finally, thefluid can also serve to constantly hydrate the tissue and prevent anycharring and associated impedance rise, as described in more detailbelow.

FIG. 1 illustrates a diagram of one exemplary fluid ablation system 100.The system includes an elongate body 102 configured for insertion into atarget volume of tissue. The elongate body can have a variety of shapesand sizes according to the geometry of the target tissue. Further, theparticular size of the elongate body can depend on a variety of factorsincluding the type and location of tissue to be treated, the size of thetissue volume to be treated, etc. By way of example only, in oneembodiment, the elongate body can be a thin-walled stainless steelneedle between about 16- and about 18-gauge (i.e., an outer diameter ofabout 1.27 mm to about 1.65 mm), and having a length L (e.g., as shownin FIG. 2) that is approximately 25 cm. The elongate body 102 caninclude a pointed distal tip 104 configured to puncture tissue tofacilitate introduction of the device into a target volume of tissue,however, in other embodiments the tip can be blunt and can have variousother configurations. The elongate body 102 can be formed from aconductive material such that the elongate body can conduct electricalenergy along its length to one or more ablation elements located along adistal portion of the elongate body. Emitter electrode 105 is an exampleof an ablation element capable of delivering RF energy from the elongatebody.

In some embodiments, the emitter electrode 105 can be a portion of theelongate body 102. For example, the elongate body 102 can be coated inan insulating material along its entire length except for the portionrepresenting the emitter electrode 105. More particularly, in oneembodiment, the elongate body 102 can be coated with 1.5 mil of thefluoropolymer Xylan™ 8840. The electrode 105 can have a variety oflengths and shape configurations. In one embodiment, the electrode 105can be a 4 mm section of a tubular elongate body that is exposed tosurrounding tissue. Further, the electrode 105 can be located anywherealong the length of the elongate body 105 (and there can also be morethan one electrode disposed along the length of the elongate body). Inone embodiment, the electrode can be located adjacent to the distal tip104. In other embodiments, the elongate body can be formed from aninsulating material, and the electrode can be disposed around theelongate body or between portions of the elongate body.

In other embodiments, the electrode can be formed from a variety ofother materials suitable for conducting current. Any metal or metal saltmay be used. Aside from stainless steel, exemplary metals includeplatinum, gold, or silver, and exemplary metal salts includesilver/silver chloride. In one embodiment, the electrode can be formedfrom silver/silver chloride. It is known that metal electrodes assume avoltage potential different from that of surrounding tissue and/orliquid. Passing a current through this voltage difference can result inenergy dissipation at the electrode/tissue interface, which canexacerbate excessive heating of the tissue near the electrodes. Oneadvantage of using a metal salt such as silver/silver chloride is thatit has a high exchange current density. As a result, a large amount ofcurrent can be passed through such an electrode into tissue with only asmall voltage drop, thereby minimizing energy dissipation at thisinterface. Thus, an electrode formed from a metal salt such assilver/silver chloride can reduce excessive energy generation at thetissue interface and thereby produce a more desirable therapeutictemperature profile, even where there is no liquid flow about theelectrode.

The electrode 105 or other ablation element can include one or moreoutlet ports 108 that are configured to deliver fluid from an innerlumen 106 extending through the elongate body 102 into surroundingtissue (as shown by arrows 109). Alternatively, the electrode 105 can bepositioned near one or more outlet ports 108 formed in the elongate body102. In many embodiments, it can be desirable to position the electrodeadjacent to the one or more outlet ports to maximize the effect of theflowing fluid on the therapy. The outlet ports 108 can be formed in avariety of sizes, numbers, and pattern configurations. In addition, theoutlet ports 108 can be configured to direct fluid in a variety ofdirections with respect to the elongate body 102. These can include thenormal orientation (i.e., perpendicular to the elongate body surface)shown by arrows 109 in FIG. 1, as well as orientations directedproximally and distally along a longitudinal axis of the elongate body102, including various orientations that develop a circular or spiralflow of liquid around the elongate body. Still further, in someembodiments, the elongate body 102 can be formed with an open distal endthat serves as an outlet port. By way of example, in one embodiment,twenty-four equally-spaced outlet ports 108 having a diameter of about0.4 mm can be created around the circumference of the electrode 105using Electrical Discharge Machining (EDM). One skilled in the art willappreciate that additional manufacturing methods are available to createthe outlet ports 108. In addition, in some embodiments, the outlet portscan be disposed along a portion of the elongate body adjacent to theelectrode, rather than being disposed in the electrode itself.

The inner lumen 106 that communicates with the outlet ports 108 can alsohouse a heating assembly 110 configured to heat fluid as it passesthrough the inner lumen 106 just prior to being introduced into tissue.Furthermore, the portion of the elongate body located distal to theelectrode 105 or other ablation element can be solid or filled such thatthe inner lumen 106 terminates at the distal end of the electrode 105.In one embodiment, the inner volume of the portion of the elongate bodydistal to the electrode can be filled with a plastic plug that can beepoxied in place or held by an interference fit. In other embodiments,the portion of the elongate body distal to the electrode can be formedfrom solid metal and attached to the proximal portion of the elongatebody by welding, swaging, or any other technique known in the art.

Fluid can be supplied to the inner lumen 106 and heating assembly 110from a fluid reservoir 112. The fluid reservoir 112 can be connected tothe inner lumen 106 via a fluid conduit 114. The fluid conduit 114 canbe, for example, a length of flexible plastic tubing. The fluid conduit114 can also be a rigid tube, or a combination of rigid and flexibletubing.

Fluid can be urged from the fluid reservoir 112 into the inner lumen 106by a pump 116. The pump 116 can be a syringe-type pump that produces afixed volume flow with advancement of a plunger (not shown). An exampleof such a pump is a Model 74900 sold by Cole-Palmer Corporation ofChicago, Ill. Other types of pumps, such as a diaphragm pump, may alsobe employed.

The pump 116 can be controlled by a power supply and controller 118. Thepower supply and controller 118 can deliver electrical control signalsto the pump 116 to cause the pump to produce a desired flow rate offluid. The power supply and controller 118 can be connected to the pump116 via an electrical connection 120. The power supply and controller118 can also be electrically connected to the elongate body 102 viaconnection 122, and to a collector electrode 124 via connection 126. Inaddition, the power supply and controller 118 can be connected to theheating assembly 110 through a similar electrical connection, asdescribed below.

The collector electrode 124 can have a variety of forms. For example,the collector electrode 124 can be a large electrode located outside apatient's body. In other embodiments, the collector electrode 124 can bea return electrode located elsewhere along the elongate body 102, or itcan be located on a second elongate body introduced into a patient'sbody near the treatment site.

In operation, the power supply and controller 118 can drive the deliveryof fluid into target tissue at a desired flow rate, the heating of thefluid to a desired therapeutic temperature, and the delivery oftherapeutic ablative energy via the one or more ablation elements, suchas electrode 105. To do so, the power supply and controller 118 canitself comprise a number of components for generating, regulating, anddelivering required electrical control and therapeutic energy signals.For example, the power supply and controller 118 can include one or morefrequency generators to create one or more RF signals of a givenamplitude and frequency. These signals can be amplified by one or moreRF power amplifiers into relatively high-voltage, high-amperage signals,e.g., 50 volts at 1 amp. These RF signals can be delivered to theablation element via one or more electrical connections 122 and theelongate body 102 such that RF energy is passed between the emitterelectrode 105 and the collector electrode 124 that can be locatedremotely on a patient's body. In embodiments in which the elongate bodyis formed from non-conductive material, the one or more electricalconnections 122 can extend through the inner lumen of the elongate bodyor along its outer surface to deliver current to the emitter electrode105. The passage of RF energy between the ablation element and thecollector electrode 124 can heat the tissue surrounding the elongatebody 102 due to the inherent electrical resistivity of the tissue. Thepower supply and controller 118 can also include a directional couplerto feed a portion of the one or more RF signals to, for example, a powermonitor to permit adjustment of the RF signal power to a desiredtreatment level.

The elongate body 102 illustrated in FIG. 1 can be configured forinsertion into a patient's body in a variety of manners. FIG. 2illustrates one embodiment of a medical device 200 having an elongatebody 202 coupled to a distal end thereof and configured for laparoscopicor direct insertion into a target area of tissue. In addition to theelongate body 202, the device 200 can include a handle 204 to allow anoperator to manipulate the device. The handle 204 can include one ormore electrical connections 206 that connect various components of theelongate body (e.g., the heating assembly and ablation element 205) to,for example, the power supply and controller 118 described above. Thehandle 204 can also include at least one fluid conduit 208 forconnecting a fluid source to the device 200.

While device 200 is one exemplary embodiment of a medical device thatcan be adapted for use in fluid enhanced ablation, a number of otherdevices can also be employed. For example, a very small elongate bodycan be required in treating cardiac dysrhythmias, such as ventriculartachycardia. In such a case, an appropriately sized elongate body canbe, for example, disposed at a distal end of a catheter configured forinsertion into the heart via the circulatory system. In one embodiment,a stainless steel needle body between about 20- and about 25-gauge(i.e., an outer diameter of about 0.5 mm to about 0.9 mm) can bedisposed at a distal end of a catheter. The catheter can have a varietyof sizes but, in some embodiments, it can have a length of about 120 cmand a diameter of about 8 French (“French” is a unit of measure used inthe catheter industry to describe the size of a catheter and is equal tothree times the diameter of the catheter in millimeters).

Therapeutic Treatment Using Fluid Enhanced Ablation

Ablation generally involves the application of high or low temperaturesto cause the selective necrosis and/or removal of tissue. There is aknown time-temperature relationship in the thermal destruction of tissueaccomplished by ablation. A threshold temperature for causingirreversible thermal damage to tissue is generally accepted to be about41° Celsius (C). It is also known that the time required to achieve aparticular level of cell necrosis decreases as the treatment temperatureincreases further above 41° C. It is understood that the exacttime/temperature relationship varies by cell type, but that there is ageneral relationship across many cell types that can be used todetermine a desired thermal dose level. This relationship is commonlyreferred to as an equivalent time at 43° C. expressed as:

t _(eq.43° C.) =∫R ^((T(t)−43°)) dt   (1)

where T is the tissue temperature and R is a unit-less indicator oftherapeutic efficiency in a range between 0 and 5 (typically 2 fortemperatures greater than or equal to 43° C., zero for temperaturesbelow 41° C., and 4 for temperatures between 41 and 43° C.), asdescribed in Sapareto S. A. and W. C. Dewey, Int. J. Rad. Onc. Biol.Phys. 10(6):787-800 (1984). This equation and parameter set representsjust one example of the many known methods for computing a thermal dose,and any of methodology can be employed with the methods and devices ofthe present invention. Using equation (1) above, thermal doses in therange of t_(eq.43° C.)=20 minutes to 1 hour are generally accepted astherapeutic although there is some thought that the dose required tokill tissue is dependent on the type of tissue. Thus, therapeutictemperature may refer to any temperature in excess of 41° C., but thedelivered dose and, ultimately, the therapeutic effect are determined bythe temporal history of temperature (i.e., the amount of heating thetissue has previously endured), the type of tissue being heated, andequation (1). For example, Nath, S. and Haines, D. E., Prog. Card. Dis.37(4):185-205 (1995) (Nath et al.) suggest a temperature of 50° C. forone minute as therapeutic, which is an equivalent time at 43° C. of 128minutes with R=2. In addition, for maximum efficiency, the therapeutictemperature should be uniform throughout the tissue being treated sothat the thermal dose is uniformly delivered.

FIG. 3 illustrates the performance profiles of several ablationtechniques by showing a simulated temperature profile achieved as afunction of distance from an ablation element, such as electrode 105.The first profile 302 illustrates the performance of RF ablation withoutthe use of fluid enhancement. As shown in the figure, the temperature ofthe tissue falls very sharply with distance from the electrode. Thismeans that within 10 mm of the ablation element the temperature of thetissue is still approximately body temperature (37° C.), far below thetherapeutic temperature of 50° C. discussed above. Furthermore, veryclose to the ablation element the temperature is very high, meaning thatthe tissue will more quickly desiccate, or dry up, and char. Once thishappens, the impedance of the tissue rises dramatically, making itdifficult to pass energy to tissue farther away from the ablationelement.

A second tissue temperature profile 304 is associated with a secondprior art system similar to that described in U.S. Pat. No. 5,431,649.In this second system, an electrode is inserted into tissue and impartsa 400 kHz RF current flow of about 525 mA to heat the tissue. Bodytemperature (37° C.) saline solution is simultaneously injected into thetissue at a rate of 10 ml/min. The resulting tissue temperature profile304 is more uniform than profile 302, but the maximum temperatureachieved anywhere is approximately 50° C. Thus, the temperature profile304 exceeds the generally accepted tissue damaging temperature thresholdspecified for one minute of therapy in only a small portion of thetissue. As described above, such a small temperature increment requiressignificant treatment time to achieve any therapeutically meaningfulresults.

A third tissue temperature profile 306 is achieved using the teachingsof the present invention. In the illustrated embodiment, an electrodeformed from silver/silver chloride is inserted into tissue and imparts a480 kHz RF current flow of 525 mA to heat the tissue. Saline solutionheated to 50° C. is simultaneously injected into the tissue at a rate of10 ml/min. The resulting temperature profile 306 is both uniform andsignificantly above the 50° C. therapeutic threshold out to 15 mm fromthe electrode. Moreover, because the temperature is uniform within thevolume, the thermal dose delivered is also uniform through the volume.

The uniform temperature profile seen in FIG. 3 can be achieved by theintroduction of heated fluid into the target tissue during applicationof ablative energy. The fluid convects the heat deeper into the tissue,thereby reducing the charring and impedance change in tissue that occursnear the ablation element, as shown in profile 302. Further, because thefluid is heated to a therapeutic level, it does not act as a heat sinkthat draws down the temperature of the surrounding tissue, as seen inprofile 304. Therefore, the concurrent application of RF energy andperfusion of heated saline solution into the tissue eliminates thedesiccation and/or vaporization of tissue adjacent to the electrode,maintains the effective tissue impedance, and increases the thermaltransport within the tissue being heated with RF energy. The totalvolume of tissue that can be heated to therapeutic temperatures, e.g.,greater than 41° C., is thereby increased. For example, experimentaltesting has demonstrated that a volume of tissue having a diameter ofapproximately 8 cm (i.e., a spherical volume of approximately 156 cm³)can be treated in 5 minutes using the fluid enhanced ablation techniquesdescribed herein. By comparison, conventional RF can only treat volumeshaving a diameter of approximately 3 cm (i.e., a spherical volume ofapproximately 14 cm³) in the same 5-minute timespan.

In addition, fluid enhanced ablation devices according to the presentinvention have a greater number of parameters that can be varied toadjust the shape of the treatment profile according to the tissue beingtreated. For example, when using the SERF ablation technique, anoperator or control system can modify parameters such as salinetemperature (e.g., from about 40° C. to about 80° C.), saline flow rate(e.g., from about 0 ml/min to about 20 ml/min), RF power (e.g., fromabout 0 W to about 100 W), and duration of treatment (e.g., from about 0min to about 10 min) to adjust the temperature profile 306. In addition,different electrode configurations can also be used to vary thetreatment. For example, although the emitter electrode 105 illustratedin FIG. 1 is configured as a continuous cylindrical band adapted for amono-polar current flow, the electrode can also be formed in othergeometries, such as spherical or helical, that form a continuous surfacearea, or the electrode may have a plurality of discrete portions. Theelectrodes may also be configured for bipolar operation, in which oneelectrode (or a portion of an electrode) acts as a cathode and anotherelectrode (or portion thereof) acts as an anode.

A preferred fluid for use in the SERF ablation technique is sterilenormal saline solution (defined as a salt-containing solution). However,other liquids may be used, including Ringer's solution, or concentratedsaline solution. A fluid can be selected to provide the desiredtherapeutic and physical properties when applied to the target tissueand a sterile fluid is recommended to guard against infection of thetissue.

Dual-Wire Heating Assembly

As described above, saline or another fluid flowing within an innerlumen of an elongate body can be heated to a therapeutic temperature bya heating assembly disposed within the inner lumen. FIG. 4 illustratesone embodiment of such an assembly. An elongate body 402 having aproximal end and a pointed distal end 404 includes an inner lumen 406.The elongate body 402 can also include at least one ablation element,such as emitter electrode 405, that is configured to deliver RF energyto tissue surrounding the elongate body 402. The electrode 405 alsoincludes one or more outlet ports 408 configured to deliver fluid fromthe inner lumen 406 into surrounding tissue.

Disposed within the inner lumen 406 is a heating assembly that includestwo wires 410, 412 that are suspended a distance apart by one or morespacers 414, 414′. The wires 410, 412 can be connected to a power sourcesuch that electrical energy can be passed between the wires through thefluid flowing in the inner lumen 406. The passage of electrical (e.g.,RF) energy through the fluid in the inner lumen 406 can cause the fluidto increase in temperature due to the natural electrical resistivity ofthe fluid, similar to the mechanism discussed above by which tissuesurrounding the elongate body can be heated using RF energy. The wires410, 412 can be formed from any conductive material, similar to thematerials discussed above in connection with the electrode 105. In oneembodiment, however, the wires 410, 412 can be formed from silver wireand can have an exposed chlorided surface between or adjacent to thespacers 414, 414′. As discussed above, these materials can participatein an ion exchange process that minimizes the voltage drop across thewire/fluid interface and prevents excessive heating of the surroundingfluid.

In order to effectively pass energy through the fluid flowing within theinner lumen 406, in an exemplary embodiment, the wires 410, 412 (or atleast the exposed portion of the wires) are prevented from coming intocontact with one another, as this can cause an electrical short. Thespacers 414, 414′ can have a variety of configurations, but in oneembodiment they can be disc-shaped members that maintain the wires 410,412 in a fixed geometric relationship with one another, i.e., at a fixeddistance apart and at a fixed orientation in space relative to oneanother. In some embodiments, the wires 410, 412 are exposed for only ashort distance located just proximal of the electrode 405 and outletports 408. As shown in FIG. 4, the wires can be exposed for a distanced₁ between the two spacers 414, 414′ that are positioned at a proximaland a distal end of a distal portion of the wires. Proximal to thespacer 414, the wires 410, 412 can be covered in an electricallyinsulating material 418 to prevent the passage of electrical energytherebetween. In addition, the wires 410, 412 can also be prevented fromdirectly contacting the elongate body 402, as an electrical short canresult from both of the wires 410, 412 simultaneously contacting theelectrically conductive elongate body. Accordingly, in some embodiments,the elongate body 402 can be lined with an insulating material 420, suchas a plastic tube, liner, or coating disposed on the inner walls of theelongate body 402.

Furthermore, the spacers 414, 414′ can be configured to occupy theentire inner diameter of the elongate body 402, or can be configured tofloat within the elongate body's inner lumen 406 by having a maximumouter diameter that is less than a diameter of the inner lumen 406. Thisconfiguration can allow the spacers 414, 414′ to move radially relativeto the central longitudinal axis of the inner lumen 406. The position ofthe spacers 414, 414′ can be fixed by configuring the spacers to have aninterference fit between the inner walls of the elongate body 402 orinsulating material 420, by adhering the spacers 414, 414′ to a portionof the elongate body using, for example, an adhesive, or by usingspokes, arms, or other surface features that extend radially outwardfrom the spacers and engage the inner wall of the inner lumen.Accordingly, the spacers 414, 414′ can also be effective to maintain thewires 410, 412 in a substantially fixed geometric relationship with theelongate body 402.

The number of spacers 414 required to maintain the wires 410, 412 inrelation with each other and/or the elongate body 402 can vary accordingto a number of factors including the length of the exposed wire section,the inner diameter of the elongate body 402, the diameter of the wires410, 412, the stiffness of the wires used, and the size of the elongatebody 402. In the embodiment illustrated in FIG. 4, two spacers 414, 414′are used to hold the wires 410, 412 apart over a distance d₁. Thedistance d₁ can vary and, in one embodiment, can be about 5 mm.Furthermore, the thickness of the spacers 414 can also be adjustedaccording to the mechanical demands required by the particularconfiguration of the elongate body 402 and wires 410, 412.

The inner lumen 406 can also house one or more temperature sensors tomonitor and assist in controlling the heating of fluid flowing withinthe inner lumen. The embodiment illustrated in FIG. 4 includes achromel-constantan fine-wire thermocouple configured to float in thefluid distal of the spacer 414′ by a distance d₂. One skilled in the artwill appreciate that thermocouples are just one example of temperaturesensors that can be employed to measure the temperature of the flowingfluid and that a variety of sensors, including thermistors and diodes,can also be used. Further, distance d₂ can vary and, in one embodiment,it can be about 10 mm. The thermocouple 422 can also be disposed adistance d₃ proximal to the electrode 405 and outlet ports 408. Whilethis distance can vary as well, in one embodiment, the distance d₃ isabout 2 mm. The distances d₁, d₂, and d₃ (and the correspondingpositions of the spacers 414, 414′) can be chosen to allow forsufficient heating of fluid flowing in the inner lumen 406, as well asto allow sufficient mixing of the heated fluid prior to flowing throughoutlet ports 408 so as to assure that the fluid being injected intotissue surrounding the elongate body 402 has a uniform temperature.However, the distances d₂ and d₃ should be minimized such that heatingof the fluid flowing within the inner lumen 406 occurs as close to theoutlet ports 408 as possible. This configuration minimizes the thermallosses and unintentional environmental heating associated withtransporting heated fluids from remote locations within a patient'sbody.

FIGS. 5A-5C illustrate the device of FIG. 4 in cross-section atlocations A, B, and C, respectively. FIG. 5A illustrates the elongatebody 402 in a portion proximal to the heating assembly. As shown in thefigure, the elongate body 402 can be lined with an insulating material420, and wires 410, 412 can also each be coated with an insulatingmaterial 418. One skilled in the art will appreciate that the insulatingmaterial 420 is not necessary where the wires 410, 412 are coated withinsulating material 418. Therefore, the insulating material 420 can bepresent along the entire length of the elongate body, as shown in FIG.5A, or can be positioned only along portions where the wires 410, 412are exposed, as described below with respect to FIG. 5C. Further, as isexplained in more detail below, insulating material need not be presentat all in some embodiments due to the isolation of the separate powersources that connect to the wires 410, 412 and the ablation element 405.Throughout any portion of the elongate body in which the wires 410, 412are coated in insulating material 418, the wires 410, 412 can be allowedto float freely in the inner lumen 406. Alternatively, one or morespacers 414 can be disposed along the length of the elongate body 402 tomaintain the wires 410, 412 in position with respect to each other andthe elongate body 402.

FIG. 5B illustrates the elongate body 402 in a portion having a spacer414. Visible in the figure is the elongate body 402, the inner lumen406, the insulating material 420, the spacer 414, and the wires 410,412. From this view, spacer 414 is revealed as a disc or cylindricalmember having two bores and at least one central lumen 502 through whichfluid can flow to bypass the spacer 414 in the event that the spaceroccupies the entire inner diameter of the elongate body 402. In otherembodiments, particularly those in which the spacer does not occupy theentire inner diameter of the elongate body 402 such that fluid can flowaround the spacer, the spacer need not have a central lumen. In oneembodiment, the spacers 414, 414′ can be formed from a single 3-lumenextruded tube that is subsequently cut into individual spacers of adesired thickness. The wires 410, 412 can be threaded through the twobores formed in the spacer 414 (shown in FIG. 5B as co-located with thewires 410, 412) and held in place by, for example, an interference fitor an epoxy adhesive.

FIG. 5C shows the elongate body 402 in a portion adjacent to the spacer414 and between spacers 414, 414′. The wires 410, 412 are exposed inthis portion and configured to heat the fluid flowing therebetween. Insuch a location within the elongate body 402, the wires 410, 412 arefree of any insulation and prevented from contacting the elongate body402 by the insulating material 420 and the restraining forces of theadjacent spacers 414, 414′.

FIG. 6 illustrates an exploded view of a heating assembly similar tothat of FIGS. 4 and 5A-5C. As shown in the figure, an inner lumen of astainless steel elongate body 602 can be lined with an insulatingmaterial 604, such as a plastic tube. A heating assembly comprising twowires 606, 608 and one or more spacers 610 can then be placed within theinner lumen such that the wires 606, 608 are prevented from coming intodirect contact with each other or the elongate body 602.

FIG. 7 illustrates an exemplary electrical circuit for delivering RFenergy to both tissue surrounding the elongate body 402 and fluidflowing through the inner lumen 406 of the elongate body 402. In theillustrated embodiment, two separate power sources 702, 704 are utilizedto deliver electrical energy including, for example, RF energy. Thepower source 702 can be connected to the two wires 410, 412 runningthrough the inner lumen 406 of the elongate body 402. By passingelectrical current through the wires, energy can be transmitted throughthe fluid flowing within the inner lumen 406 between the exposedportions of the wires 410, 412.

The power source 704 can be connected to both the elongate body 402 anda collector electrode 124. The collector electrode can be locatedremotely on a patient's body, for example, placed under a patient's backon an operating table. As discussed above, in other embodiments, thecollector electrode 124 can be co-located on the elongate body 402 or itcan be located on a second elongate body positioned nearby the elongatebody 402. One skilled in the art will appreciate that positioning thecollector electrode 124 on the elongate body 402 requires isolating theemitter electrode 405 from the collector electrode. This can beaccomplished in a variety of manners including, for example, by formingthe elongate body 402 from a non-conducting material and placing the twoelectrodes on the surface of the elongate body 402. In such anembodiment, the power source 704 can be connected to the two electrodesby any suitable electrical connection, such as wires extending throughthe inner lumen of the elongate body 402 or along its outer surface.

Referring back to the figure, the power source 704 can deliver RF energyfrom the electrode 405 to the collector electrode 124 by passingelectrical current through the elongate body 402. The two power sources702, 704 do not share a common electrical ground and therefore remainelectrically isolated from one another. This ensures that power from thesource 702 heats only saline flowing within the elongate body 402, whilepower from the source 704 heats only tissue surrounding the elongatebody 402. The spacers and insulating materials discussed above areutilized to prevent a short between the two wires 410, 412 that canresult from the wires touching each other or simultaneously contactingthe elongate body 402. One skilled in the art will appreciate that avariety of combinations of spacers and insulating materials covering thewires and/or the inner walls of the elongate body can be used to preventsuch an electrical short circuit.

In an exemplary embodiment, as saline solution is pumped through theelongate body's inner lumen 406, the saline can be heated above bodytemperature by the power source 702, preferably to between about 50° C.and about 70° C. This can be accomplished by delivering RF energy to thefluid within the inner lumen 406 via the wires 410, 412. For example,typical fluid enhanced ablation therapy operating parameters involve theapplication of 20 volts or more to the wires 410, 412. In someembodiments, the applied voltage can go as high as 120 volts and, insome embodiments, can be about 30 volts (e.g., 31.25 volts in oneembodiment). The heated, flowing saline solution can be subsequentlyinjected into tissue surrounding the elongate body 402 via the outletports 408 at a variety of flow rates. For example, in some embodimentsfluid can be ejected from the elongate body 402 at a flow rate of about10 ml/min. The delivery of heated fluid can be done independently or inconjunction with the delivery of ablative energy from the power source704. The operating parameters of fluid enhanced ablation therapy canvary according to a number of factors, including the desired therapeuticeffect, the geometry and tissue properties of the treatment volume, etc.By way of example, in one embodiment ablation therapy conducted in apatient's liver can heat saline to 50° C. using 40 watts of power anddeliver the saline at 10 ml/min for about 5 minutes. By way of furtherexample, ablation therapy using these same parameters can be deliveredfor only about 90 seconds when treating cardiac tissue. While theparticular properties of the intended treatment site will ultimatelygovern the selected operating parameters, fluid enhanced ablationtherapy typically involves the delivery of saline at a rate betweenabout 0 and about 20 ml/min. The saline is typically heated to betweenabout 50° C. and 80° C. using up to 80 watts of power and up to 120volts. Fluid heated according to these exemplary operating parameterscan be combined with electrical energy delivered directly to the tissueto conduct ablation therapy. In some embodiments, up to 100 watts ofpower can be applied to the tissue from, for example, an emitterelectrode.

Single-Wire Heating Assembly

A second embodiment of the heating assembly 110 is illustrated in FIG.8. This embodiment uses a single wire in combination with a conductiveelongate body or a conductive tube placed within the elongate body todeliver RF energy to fluid flowing within an inner lumen of the elongatebody. This heater design can have advantages in embodiments where smallelongate bodies are used to access, for example, areas of a patient'sheart in treating cardiac dysrhythmias such as ventricular tachycardia.Further, this configuration can provide more uniform heating of fluidflowing through the inner lumen of the elongate body, as discussedbelow. Still further, one skilled in the art will appreciate that thedual-wire and single-wire assemblies can include any combination offeatures disclosed herein with respect to each embodiment.

As shown in the figure, the elongate body 802 includes a proximal endand a pointed distal tip 804, and includes at least one ablationelement, such as electrode 805, disposed along the length thereof. Theelongate body 802 also includes an inner lumen 806 that is in fluidcommunication with one or more outlet ports 808 formed in the electrode805. The elongate body 802 can be formed from similar materials as theelongate body 102 discussed above, i.e., electrically conductivematerials capable of passing current from a power source to theelectrode 805.

Disposed within the inner lumen 806 of the elongate body 802 can be awire 810 configured to deliver RF energy to fluid flowing within theinner lumen via an exposed portion extending a distance d₁ between twospacer elements 812, 812′. The spacer elements 812, 812′ can hold thewire 810 in a substantially fixed geometric relationship with theelongate body 802 and can be formed from an electrically insulatingmaterial such as a plastic. By maintaining a substantially fixedgeometric relationship, the spacers 812, 812′ can prevent the exposedportion of the wire 810 from directly contacting the elongate body 802and causing an electrical short. Note that the wire 810 can be coated inan insulating material (not shown) similar to insulating material 418along any portions located proximal to the spacer 812.

The distance d₁ separating the spacers 812, 812′ can vary according tothe desired heating capacity, power source, wire diameter, and elongatebody size in a particular embodiment. In one embodiment, the elongatebody can be a 25-gauge thin-walled needle having an inner diameter ofabout 0.4 mm. A wire having an outer diameter less than the innerdiameter of the elongate body can have an exposed portion where d₁ canbe about 2 mm. In one embodiment, the wire can have an outer diameter ofabout 0.125 mm. The exposed portion of the wire 810 can be located justproximal to the electrode 805 and its outlet ports 808, but somedistance should be left separating the components so that fluid beingheated by the wire 810 has time to sufficiently mix and reach a moreuniform temperature before being introduced into tissue surrounding theelongate body.

Similar to the first embodiment discussed above, one or more temperaturesensors 814 can also be disposed within the inner lumen 806 to aid incontrolling the heating of fluid flowing through the inner lumen. Forexample, a temperature sensor 814 can be positioned between the distalend of the wire 810 and the proximal end of the electrode 805. That is,the temperature sensor 814 can be positioned a distance d₂ beyond thedistal end of the wire 810, and a distance d₃ before the proximal end ofthe electrode 805. In some embodiments, the distance d₂ can be about 1mm and the distance d₃ can be nearly 0 mm. The temperature sensor can beany of a variety of sensors, and in some embodiments can be a fine-wirechromel-constantan thermocouple known in the art.

FIGS. 9A and 9B illustrate the elongate body of FIG. 8 in cross-sectionat locations A and B, respectively. As shown in FIG. 9A, when the wire810 is exposed between the spacers 812, 812′, it is free to passelectrical energy to the elongate body 802, thereby heating any fluid inthe inner lumen 806. Of note is the fact that the elongate body 802 isnot lined with an insulating material, as described above. Rather, theelongate body 802 in this embodiment serves as the second electrode toreceive energy from the wire 810. However, in some embodiments theelongate body 802 can be lined with an insulating material proximal tothe spacer 812.

FIG. 9B illustrates the elongate body at a position in which the wire810 is protected from contacting the elongate body 802 by a spacer 812.The spacer 812 can be capable of maintaining the wire 810 in a positionsubstantially coaxial with a longitudinal axis of the elongate body. Asshown in the figure, the spacer 812 need not occupy the entire diameterof the inner lumen 806. Rather, the spacer 812 can be formed in avariety of sizes and, in some embodiments, may occupy substantially allof the available space in the inner lumen 806 while, in otherembodiments, it may be significantly smaller. In embodiments in whichthe spacer 812 does occupy substantially all of the inner lumen 806, itmay be necessary to form the spacer with one or more passages such thatfluid can flow around the spacer, similar to the central lumen 502 ofthe spacer 414 described above. One skilled in the art will appreciatethat the spacer 812 can be formed as an extruded tube, similar to thespacer 414. Such an extrusion can be formed with one or more lumens thatcan serve as passageways for fluid. In addition, the wire 810 can becoated in an insulating material over portions where energy transfer isundesirable. For example, the wire 810 can be coated with an insulatingmaterial over the portion of the wire extending proximally from thespacer 812.

FIG. 10 illustrates one embodiment of an electrical circuit forindependently delivering RF energy to fluid flowing in the inner lumen806 of the elongate body 802, as well as to tissue surrounding theelongate body. As shown in the figure, dual power sources 1002, 1004 areused to deliver energy to the fluid within the inner lumen 806 and thetissue surrounding the elongate body 802, similar to the circuitillustrated in FIG. 7. However, in the illustrated embodiment, thecircuits formed by each power source 1002, 1004 share the elongate body802 as a common electrode. In other words, power source 1002, which isconfigured to deliver RF energy to fluid flowing within the inner lumen806, is connected to the wire 810 disposed within the inner lumen 806and to the elongate body 802 itself. The elongate body 802, then, servesas an electrode for the power source 1002. The power source 1004, on theother hand, is connected to the elongate body 802 and the collectorelectrode 124. Accordingly, the power source 1004 can deliver RF energyfrom the electrode 805 into tissue surrounding the elongate body 802. Asa result of the fact that the two power sources 1002, 1004 are onlyconnected via the elongate body 802 (i.e., only connected at a singlepoint without a return connection), the power sources are able tooperate independently and simultaneously without any current flowingtherebetween.

FIGS. 11A, 11B, and 12 illustrate alternative embodiments of the spacer812. As described above, the spacer 812 can be formed as an extrusion ofan insulating material, such as a polymer material, that cansubsequently be affixed to the wire 810 using, for example,overjacketing extrusion with pressure tooling or using an adhesive. Thespacer 812 can have a cylindrical shape to match the cylindrical shapeof the inner lumen 806 and, in embodiments in which a non-cylindricalelongate body is used, the spacer 812 can be formed into any shapecorresponding to the inner lumen 806. The spacer 812 can, however,assume other configurations as well. For example, the spacer 812 can beformed as one or more features attached to the wire 810 along itslength. FIG. 11A shows one embodiment of such a spacer. The conductingwire 1102 can have formed thereon a number of longitudinally extendingprotrusions or ridges 1104 that are formed from an insulating material.The protrusions 1104 act similarly to the spacer 812 and prevent thewire 1102 from directly contacting the elongate body 802. Theseprotrusions, unlike the spacer 812, do not completely surround the wire1102, so they can extend along the entire exposed portion of the wire1102 without preventing the passage of electrical energy through fluidflowing over the wire. As a result, energy can be passed between thewire 1102 and the elongate body 802 along any desired length withoutrequiring individual spacer elements to hold an exposed length of thewire (e.g., spacers 812, 812′).

FIG. 11B illustrates a cross-sectional view of the wire 1102, moreclearly showing the four protrusions 1104 formed thereon. One skilled inthe art will appreciate that any number of protrusions or ridges may beformed around the circumference of the wire 1102, but the number shouldbe sufficient to prevent direct electrical contact with the elongatebody 802 while not completely enclosing the surface area of the wire1102. Furthermore, the protrusions can be formed in a variety of sizes.For example, the protrusions may be formed with a height that reaches tothe inner walls of the elongate body 802, thereby preventing the wire1102 from moving within the inner lumen 806. In other embodiments, theprotrusions may have a lesser height that allows the wire 1102 to moveradially within the inner lumen 806 while preventing direct contactbetween an exposed portion of the wire and the inner walls of theelongate body 802.

The protrusions 1104 can be formed on the wire 1102 in a variety ofmanners. For example, the protrusions can be extruded and subsequentlyapplied to the surface of the wire 1102. Alternatively, the wire can beformed with an insulating coating as known in the art, and the coatingcan be selectively removed through an ablation process such that onlythe protrusions 1104 of insulating material remain. Alternatively, thewire can be formed and then the insulating protrusions applied usingoverjacketing extrusion. In addition, the protrusions 1104 can be formedin a variety of shapes other than the longitudinally extending ridgesshown in FIGS. 11A and 11B. For example, FIG. 12 illustrates an exampleof a wire 1202 that has an insulating material coating 1204 in the shapeof an auger or corkscrew. Such a shape can be created using, forexample, the selective ablation process described above to removeportions of an insulating coating formed over the wire 1202.

Design Advantages

The various embodiments of the heating assembly 110 described hereinprovide a number of advantages for fluid enhanced ablation systems. Forexample, the ability of the spacers to hold one or more wires in asubstantially fixed geometric relationship with a second wire and/or theelongate body itself enables the reliable production of a fluid enhancedablation device or system. This is because any power source utilized todeliver RF energy to fluid flowing within the inner lumen of an elongatebody can be configured to effectively heat only a limited range ofresistances. Therefore, in order to obtain consistent performance, theresistance encountered by the power source must remain within thiseffective heating range of the power source. The electrical resistanceof a structure depends generally on both the specific resistivity of thematerials that comprise the structure, as well as its geometry. Thespecific resistance of a fluid flowing within the elongate body 102, aswell as that of the materials used in forming the elongate body and theheater 110 are known. This means the remaining unknown variation inresistance is likely to come from variations in the geometry of thecomponents (e.g., movement of the electrodes with respect to each other,etc.). By minimizing this geometric variation, the spacers can enablemore consistent performance of the fluid enhanced ablation system.

Furthermore, the spacers can allow for a simplified manufacturingprocess in assembling fluid enhanced ablation devices. As mentionedabove, the spacers can easily be extruded and cut to size or formed byselectively removing coatings from electrically conductive wires, etc.These components can then be easily placed within the inner lumen of anelongate body and positioned using, for example, interference fits andadhesive materials, or they can be allowed to float within the innerlumen.

Yet another benefit of the designs disclosed herein is that they providefor the uniform heating of fluid flowing through the inner lumen of anelongate body. One problem commonly encountered with heating fluid bypassing RF energy between electrodes is the localized boiling of fluidthat occurs in areas where the current density is particularly high. Forexample, if two electrodes (e.g., two wires) are located very close toeach other, there can be a disproportionately high current flux in thesmall space between the electrodes when compared to the current fluxseen in other areas of the volume surrounding the electrodes. As aresult, fluid flowing through the small space between the electrodes canbecome super-heated and may boil. This is undesirable because it couldcause medical complications in a patient (e.g., by introducing gasbubbles into the blood flow of the heart). Accordingly, it is desirableto achieve the most uniform heating of fluid possible so as to minimizeboiling during heating of fluid within the inner lumen.

The spacers disclosed herein aid in the uniform heating of fluid withinthe inner lumen by maintaining the electrodes used to heat the fluid ina relationship that minimizes areas of disproportionately high currentflux between the electrodes. This concept is illustrated in FIGS. 13Aand 13B, which depict the dual-wire and single-wire heating assembliesin cross-section with simulated lines of current passing between the twoelectrodes. FIG. 13A, which illustrates the dual-wire heating assemblyshown in FIG. 5C, demonstrates that by maintaining the wires in aseparated configuration, the variations in current flux throughout theinner lumen 406 can be minimized. The single-wire heating elementdepicted in FIGS. 9A and 13B provides an even more uniform currentdistribution within the inner lumen. As shown in FIG. 13B, current canpass uniformly from around the entire circumference of the wire 810 intothe elongate body 802. This results in very uniform heating of the fluiddisposed within the inner lumen 806.

Applicability to Other Forms of Ablation

Those that are knowledgeable in the art will recognize that the heatingmechanism for producing hyperthermia within the target tissue sufficientto destroy it can include other forms of energy. Ultrasonic vibrationalenergy is known to be absorbed by tissue and converted to heat, as ismicrowave and light wave electromagnetic energy. Alternative embodimentsof may employ ultrasonic transducers, microwave antennas, or light wavediffusers as emitters disposed in the distal end of an elongate body.Light wave electromagnetic energy can fall in a range spanning visible,near-infrared, infrared, and far-infrared radiation, and can begenerated by filaments, arc lamps, lasers of various forms (e.g.,diodes, semiconductors, or pumps), or by other means. Similarly, thedistal end of an elongate body can be adapted to include a heatingmechanism such as a resistive wire for heating the tissue throughconduction. Regardless of the type of ablation element utilized, theinjection of heated liquid into the tissue proximate any of theseablation elements will improve the ability of each device to heat largevolumes of tissue. Accordingly, the heating assemblies disclosed hereincan be applicable to devices that utilize any of these alternativeablative energy sources. It is also recognized that the delivery devicecan be any standard medical delivery device, depending on the tissue tobe treated. Alternative embodiments can include metallic or nonmetallicneedles, sheaths, or introducers.

Methods of Use

As described above, the various embodiments of the devices and systemsdisclosed herein can be utilized in a variety of procedures to treat anumber of medical conditions. For example, medical devices as disclosedherein can be configured for insertion into a target volume of tissuedirectly during an open surgical procedure or during percutaneousablation therapy. Alternatively, the medical devices can be configuredto be passed through one or more layers of tissue during a laparoscopicor other minimally invasive procedure. Furthermore, the devices can beconfigured for introduction into a patient via an access port or otheropening formed through one or more layers of tissue, or via a naturalorifice (i.e., endoscopically). Depending on the device employed,delivery may be facilitated by directly inserting the elongate body asshown in FIG. 2, or by introducing a catheter containing an elongatebody through, for example, a patient's circulatory system. Followingdelivery to a treatment site, a portion of a surgical device, e.g., adistal portion of the elongate body 102, can be inserted into a targettreatment volume such that an ablation element is disposed within thetreatment volume. In some embodiments, the ablation element can bepositioned near the center of the treatment volume.

Once the device is positioned within the treatment volume, fluid can bedelivered through the device into the treatment volume. The heatingassemblies disclosed herein can be utilized to deliver fluid at atherapeutic temperature, as described above. In addition, the one ormore ablation elements can be activated to simultaneously delivertherapeutic energy, such as RF energy, into the tissue in the treatmentvolume. In some embodiments, however, the one or more ablation elementsneed not be activated, and therapy can be administered by deliveringheated fluid from the elongate body alone. After a period of time, ordepending on one or more feedback indications (e.g., a reading from atemperature sensor disposed within the treatment volume), the ablationelement can be deactivated along and the flow of fluid into the volumecan be stopped. The device can then be removed and/or repositioned ifadditional therapy is required.

Sterilization and Reuse

The devices disclosed herein can be designed to be disposed after asingle use, or they can be designed for multiple uses. In either case,however, the device can be reconditioned for reuse after at least oneuse. Reconditioning can include any combination of the steps ofdisassembly of the device, followed by cleaning or replacement ofparticular pieces, and subsequent reassembly. In particular, the devicecan be disassembled, and any number of the particular pieces or parts ofthe device can be selectively replaced or removed in any combination.Upon cleaning and/or replacement of particular parts, the device can bereassembled for subsequent use either at a reconditioning facility or bya surgical team immediately prior to a surgical procedure. Those skilledin the art will appreciate that reconditioning of a device can utilize avariety of techniques for disassembly, cleaning/replacement, andreassembly. Use of such techniques, and the resulting reconditioneddevice, are all within the scope of the present invention.

For example, the surgical devices disclosed herein may be disassembledpartially or completely. In particular, the elongate body 202 of themedical device 200 shown in FIG. 2 may be removed from the handle 204,or the entire handle and elongate body assembly may be decoupled fromthe electrical and fluid connections 206, 208. In other embodiments,solely the distal portion of the elongate body 202 (e.g., only theportion that extends into a patient's body) can decouple from a proximalportion that can remain connected to the handle 204. In yet anotherembodiment, the handle, elongate body, and connections may be removablycoupled to a housing that contains, for example, the fluid reservoir,pump, and power supply and controller shown in FIG. 1.

Preferably, the devices described herein will be processed beforesurgery. First, a new or used instrument can be obtained and, ifnecessary, cleaned. The instrument can then be sterilized. In onesterilization technique, the instrument is placed in a closed and sealedcontainer, such as a plastic or TYVEK bag. The container and itscontents can then be placed in a field of radiation that can penetratethe container, such as gamma radiation, x-rays, or high-energyelectrons. The radiation can kill bacteria on the instrument and in thecontainer. The sterilized instrument can then be stored in the sterilecontainer. The sealed container can keep the instrument sterile until itis opened in the medical facility.

In many embodiments, it is preferred that the device is sterilized. Thiscan be done by any number of ways known to those skilled in the artincluding beta or gamma radiation, ethylene oxide, steam, and a liquidbath (e.g., cold soak). In certain embodiments, the materials selectedfor use in forming components such as the elongate body may not be ableto withstand certain forms of sterilization, such as gamma radiation. Insuch a case, suitable alternative forms of sterilization can be used,such as ethylene oxide.

All papers and publications cited herein are hereby incorporated byreference in their entirety. One skilled in the art will appreciatefurther features and advantages of the invention based on theabove-described embodiments. Accordingly, the invention is not to belimited by what has been particularly shown and described, except asindicated by the appended claims.

1. An ablation device, comprising: an elongate body having proximal anddistal ends, an inner lumen extending through the elongate body, and atleast one outlet port formed in the elongate body configured to deliverfluid to tissue surrounding the elongate body; at least one wireextending through the inner lumen, the at least one wire beingconfigured to heat fluid flowing through the inner lumen; and at leastone spacer disposed within the inner lumen, the at least one wireextending through the at least one spacer such that the at least onespacer is effective to maintain an adjacent portion of the at least onewire in a substantially fixed geometric relationship with the innerlumen.
 2. The ablation device of claim 1, further comprising an ablationelement disposed along a length of the elongate body adjacent to the atleast one outlet port, the ablation element being configured to heattissue surrounding the ablation element when the elongate body isinserted into tissue.
 3. The ablation device of claim 2, wherein the atleast one wire and the at least one spacer are positioned proximal ofthe ablation element.
 4. The ablation device of claim 1, wherein the atleast one spacer comprises a first spacer positioned at a proximal endof a distal portion of the at least one wire, and a second spacerpositioned at a distal end of the distal portion of the at least onewire.
 5. The ablation device of claim 4, wherein the first and secondspacers are positioned a distance apart from one another, and whereinthe distance is about 5 mm.
 6. The ablation device of claim 1, whereinthe at least one spacer comprises a disc-shaped member and the at leastone wire comprises first and second wires configured to extend throughfirst and second bores in the spacer.
 7. The ablation device of claim 1,wherein the at least one spacer prevents the at least one wire fromcontacting the inner lumen.
 8. The ablation device of claim 1, whereinthe at least one spacer has a maximum outer diameter that is less than adiameter of the inner lumen such that the at least one spacer can moveradially within the inner lumen.
 9. The ablation device of claim 1,wherein the at least one spacer has a maximum outer diameter equal to adiameter of the inner lumen such that the at least one spacer cannotmove radially within the inner lumen.
 10. The ablation device of claim4, wherein a portion of the at least one wire extending between thefirst and second spacers is configured to heat fluid flowing through theinner lumen.
 11. The ablation device of claim 10, wherein the first andsecond spacers are positioned a distance apart from one another, andwherein the distance is about 2 mm.
 12. The ablation device of claim 1,wherein the at least one wire is insulated proximal to the at least onespacer.
 13. The ablation device of claim 1, wherein the inner lumen islined with an insulating layer to prevent the at least one wire fromcontacting an inner wall of the elongate body.
 14. The ablation deviceof claim 1, wherein the at least one spacer is configured to maintainthe at least one wire in a position substantially coaxial with alongitudinal axis of the elongate body.
 15. The ablation device of claim1, wherein the at least one spacer comprises at least one protrusionformed on the at least one wire.
 16. The ablation device of claim 1,further comprising at least one temperature sensor disposed within theinner lumen distal to the at least one spacer and configured to measurea temperature of the fluid flowing through the inner lumen.
 17. Theablation device of claim 16, further comprising a second temperaturesensor disposed within the inner lumen proximal to the at least onespacer and configured to measure a temperature of the fluid flowingthrough the inner lumen.
 18. The ablation device of claim 16, whereinthe at least one temperature sensor is a thermocouple.
 19. The ablationdevice of claim 16, wherein the at least one temperature sensor isseparated from the at least one spacer by a distance of about 10 mm. 20.The ablation device of claim 16, wherein the at least one temperaturesensor is separated from the at least one spacer by a distance of about2 mm.
 21. A method of ablating tissue, comprising: inserting an elongatebody into a tissue mass; delivering fluid through an inner lumen of theelongate body, the fluid flowing through at least one outlet port in theelongate body and into the tissue mass; and delivering energy through atleast one wire extending through the inner lumen to heat the fluidwithin the lumen to a predetermined temperature.
 22. The method of claim21, wherein delivering energy through at least one wire comprisespassing energy between two or more wires extending through the innerlumen.
 23. The method of claim 21, wherein delivering energy through atleast one wire comprises passing energy between one or more wires andthe elongate body.
 24. The method of claim 21, wherein delivering energythrough at least one wire comprises passing energy between one or morewires and a conductive tube contained within the elongate body.
 25. Themethod of claim 21, further comprising delivering energy into the tissuemass from at least one ablation element positioned adjacent to the atleast one outlet port.
 26. A method for manufacturing a plurality ofablation devices, comprising: forming a first ablation device bypositioning a first heating assembly within a first elongate body, thefirst heating assembly having at least one wire extending through atleast one spacer; and forming a second ablation device by positioning asecond heating assembly within a second elongate body, the secondheating assembly having at least one wire extending through at least onespacer; wherein an electrical resistance of the first heating assemblyis substantially identical to an electrical resistance of the secondheating assembly.
 27. An ablation device, comprising: an elongate bodyhaving proximal and distal ends, an inner lumen extending through theelongate body, and at least one outlet port formed in the elongate bodyconfigured to deliver fluid to tissue surrounding the elongate body; aheating assembly comprising at least two wires extending through theinner lumen, the at least two wires being configured to heat fluidflowing through the inner lumen; and at least one spacer disposed withinthe inner lumen, the at least two wires extending through the at leastone spacer such that the at least one spacer is effective to maintainthe at least two wires in a substantially fixed geometric relationshipwith each other.
 28. The ablation device of claim 27, further comprisingan ablation element disposed along a length of the elongate bodyadjacent to the at least one outlet port, the ablation element beingconfigured to heat tissue surrounding the ablation element when theelongate body is inserted into tissue.